implement Dark Mode Toggle in React Email Preview

[KlosterfrauDG]

Goals

1. Restore the previously available dark/light mode toggle functionality in the React Email preview interface
2. Allow developers to test how their emails appear in both light and dark environments without switching browser settings
3. Improve developer experience when testing email templates for different color schemes

Background

Prior versions of React Email included a convenient dark/light mode toggle in the preview interface, which was removed in more recent versions. This feature was valuable for developers who need to test how their email templates render in different viewing environments.

Proposal

Re-implement the dark/light mode toggle in the React Email preview interface with these considerations:

UI Implementation:

• Add a simple sun/moon toggle icon in the top toolbar
• Ensure the toggle affects only the preview pane, not the code editor
• Preserve the toggle state between sessions

[gabrielmfern]

We're actually planning to release a dark mode toggle this quarter following on the work that we already had done from a community member. We only had the toggle in canary IIRC, never released to a stable version because it could be misleading considering that email clients each have a different way of handling dark mode